As size reduction in digital cameras, cellular phones with a camera and the like is strongly required, size reduction in an image forming optical system used for these devices is also strongly required, Patent documents 1 and 2 disclose image forming optical systems used for solid-state image sensors of digital cameras, cellular phones with a camera, scanners and the like. In order to realize size reduction in an image forming optical system, there exists a method to reduce a distance from a lens to an image plane or a method to make the system wide-angle. However, these methods raise problems of an increase in angles of incidence to the image plane of the solid-state image sensor and lager chromatic aberration, astigmatism, field curvature and distortion. An increase in angles of incidence to the image plane of the solid-state image sensor prevents efficient image forming on the solid-state image sensor. Further, lager chromatic aberration, astigmatism, field curvature and distortion deteriorate resolution and picture quality of the imaging device.    Patent document 1: JP2007-286153A    Patent document 2: JP2007-219079A